The present invention relates generally to communications and more particularly to system and methods for wireless communications.
As the Internet continues its development and workers and consumers increasingly rely on data networking to assist their day-to-day tasks, a need arises to extend network connectivity to locations where there is no convenient connection to a wired infrastructure. Workers desire to send and receive email and access the Internet and their corporate intranet even when they are away from their workstation. Consumers wish to establish home networks without costly and cumbersome wiring. Accordingly, wireless communication standards have evolved including the IEEE 802.11 family.
The current IEEE 802.11a standard allows for wireless communications at speeds between 6 Mbps and 54 Mbps on a given specified carrier frequency. However, in a situation where multiple clients or subscriber units interact with a central access point, it is the case that only one access point to subscriber unit communication can take place at once on a given carrier frequency. If more than one user attempts to transmit at a time, a packet collision will occur. It would be desirable to extend IEEE 802.11a data carrying capacity by allowing for simultaneous multiple transmissions in the same frequency.
One known way of allowing such multiple parallel transmissions is the use of MIMO (Multiple Input Multiple Output) techniques to provide SDMA (Spatial Division Multiple Access). MIMO techniques take advantage of multiple antennas or multiple polarizations of the same antenna at the transmitter or receiver to access multiple channel inputs or channel outputs and thereby define multiple spatial subchannels that occupy the same bandwidth but nonetheless are capable of carrying independent data streams. The delineation of the multiple spatial subchannels may involve weighting of the antenna inputs of the transmitter or weighting of the antenna outputs at the receiver end. For further information on MIMO techniques, see U.S. Pat. No. 6,377,631. SDMA exploits the multiple spatial subchannels provided by MIMO processing to carry independent data streams to or from multiple transceivers.
It is desirable to apply MIMO-based SDMA techniques to IEEE 802.11 systems to increase data carrying capacity, but there are obstacles to overcome. The physical and media access control (MAC) layers defined by IEEE 802.11 assume that only one unit is transmitting at a time in a given carrier frequency. SDMA techniques rely on a certain amount of coordination of received power levels and timing among multiple simultaneous transmitters and 802.11 makes no provision for this type of coordination. Furthermore, 802.11 MAC operation typically assumes that an acknowledgement will be sent back after transmission of a single packet. However, in SDMA two packets of unequal length may commence transmission simultaneously and the acknowledgement of the shorter packet cannot commence immediately because the transmission of the longer packet continues to occupy the shared channel.
What is needed are systems and methods for applying MIMO and SDMA techniques to 802.11 operation and thereby allow for simultaneous transmissions by multiple users and increased system capacity.